Driver circuit and method for driving a capacitive load

ABSTRACT

An apparatus and method for a voltage driver circuit where a path between a ground node and an output node includes an inductor and/or another storage element and an energy transfer circuit composed of at least two switching elements (e.g., transistors) and at least two valve elements (e.g. diodes). The energy transfer circuit operates to discharge a capacitive load (e.g. output capacitor) into the storage element and facilitate transition from low to high voltage while increasing efficiency. An additional path to ground may be included via another switching element to prevent crosstalk and hold the output at ground potential.

BACKGROUND

Electronic devices, especially tablets or smart phones, may accept inputvia handheld peripheral devices, such as a pen or stylus, and may thenact as host devices to the peripheral devices. The stylus may be heldmanually by a user in relation to a display screen (e.g. touch screen)of a digitizer to provide input to the electronic device. Positions ofthe stylus over the display screen are correlated with virtualinformation portrayed on the display screen. Position detection and datatransfer (i.e. communication) is achieved via capacitive couplingbetween the stylus and the display screen of the digitizer (and viceversa). More specifically, a driver circuit is configured to generatebinary information (bits) by applying a voltage on a tip electrode ofthe stylus to generate a current through the capacitance between the tipelectrode and the display screen of the digitizer. This current can besensed by the digitizer. Similarly, such a driver circuit may as well beprovided at the digitizer to transfer data via the tip electrode to thestylus.

SUMMARY

This Summary is provided to introduce a selection of concepts insimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter. Nor is theclaimed subject matter limited to implementations that solve any or allof the disadvantages noted herein.

The disclosure in some embodiments relates to a driver circuit forsupplying a drive signal via an output terminal to a capacitive load,wherein the driver circuit comprises:

an energy storage element; and

an energy transfer circuit configured to transfer electric charge fromthe capacitive load to the energy storage element at a first edge of thedrive signal and to transfer electric charge from the energy storageelement to the capacitive load fat a second edge of the drive signal.

According to an aspect of some embodiments, a stylus comprises thedriver circuit, wherein the driver circuit is configured to supply thedrive signal to a tip electrode of the stylus.

According to another aspect, a host device comprises the above drivercircuit, wherein the driver circuit is configured to supply the drivesignal to a sensor array of a touch-sensitive display.

According to a further aspect, a method of supplying a drive signal viaan output terminal to a capacitive load comprises controlling an energytransfer circuit to transfer electric charge from the capacitive load toan energy storage element at a first edge of the drive signal and totransfer electric charge from the energy storage element to thecapacitive load at a second edge of the drive signal.

The enhanced driver circuit and drive control method provide highefficiency for high voltage drivers with high capacitive loads to reducepower consumption substantially.

Unless otherwise defined, all technical and/or scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art. Although methods and materials similar or equivalentto those described herein can be used in practice or testing ofembodiments of the disclosure, example methods and/or materials aredescribed below. In addition, the materials, methods, and examples areillustrative only and are not intended to be necessarily limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

To assist understanding of the present disclosure and to show howembodiments of such may be put into effect, reference is made, by way ofexample only, to the accompanying drawings in which:

FIG. 1 is a schematic block diagram of an example system comprising astylus and a host device,

FIG. 2 is a schematic circuit diagram of an example driver circuit witha push-pull circuit for applying a tip voltage of a stylus,

FIG. 3 is a schematic waveform diagram with example signals for thedriver circuit,

FIG. 4 is a schematic block diagram of an example driver circuit with anenergy transfer circuit for improved efficiency,

FIG. 5 is a schematic flow diagram of an example driver procedure forcontrolling the energy transfer circuit,

FIG. 6 is a schematic circuit diagram of a driver circuit with a firstexample of an enhanced push-pull circuit for improved efficiency,

FIG. 7 is a schematic waveform diagram with example signals for thefirst example of the enhanced push-pull circuit,

FIG. 8 is a waveform diagram with example measured signals of the firstexample of the enhanced push-pull circuit at a falling edge of a tipvoltage of a stylus,

FIG. 9 is a waveform diagram with example measured signals of the firstexample of the enhanced push-pull circuit at a rising edge of the tipvoltage of the stylus,

FIG. 10 is a schematic circuit diagram of a driver circuit with a secondexample of the enhanced push-pull circuit for improved efficiency andreduced crosstalk,

FIG. 11 is a schematic waveform diagram with example signals of thesecond example of the enhanced push-pull circuit,

FIG. 12 is a waveform diagram with example measured signals of thesecond example of the enhanced push-pull circuit at a falling edge of atip voltage of a stylus,

FIG. 13 is a waveform diagram with example measured signals of thesecond example of the enhanced push-pull circuit at a rising edge of thetip voltage of the stylus, and

FIG. 14 is a waveform diagram with example measured signals of thesecond example of the enhanced push-pull circuit over a longer timeperiod until a steady state is reached.

DETAILED DESCRIPTION

The present disclosure is directed to a driver circuit for driving acapacitive load, which may be provided in a stylus and/or a touch screencontroller of a digitizer or any other electronic device which isconfigured to supply an output signal to a capacitive load.

A hand-held stylus peripheral (“stylus”) for use with electronicdevices, including smart phones, tablets, watches, desktop computers,gaming devices, wearable device, tele-visions, video conferencingsystems, etc. may be used to communicate user input to an electronicdevice (“host device”). Some host devices include a display with abuilt-in digitizer to sense signals transmitted from the stylus (e.g.,an “active stylus”). In these electronic devices, a user interacts withthe digitizer system by positioning and moving the stylus over a sensingsurface of the system, e.g., a tablet and/or a touch screen. Theposition of the stylus with respect to the sensing surface is tracked bythe digitizer system and interpreted as a user command. In sometechnologies, the position of the stylus can be determined based ondetection of capacitive coupling between an electrode of the stylus andone or more electrodes of the digitizer. For example, the device displaymay include a digitizer with a plurality of X and Y oriented conductorsor a resistive film to receive signals transmitted from the electrode ofthe active pen. To accurately identify tip position, the transmittingelectrode is, in some technologies, physically positioned within awriting tip of the stylus.

A stylus can be classified as a passive stylus or an active stylus. Thepassive stylus utilizes sensing methods based on changes in thecapacitive coupling between sensor electrodes deposited on atouch-screen sensor and an input object, such as a rubber-tipped stylusor figure. In contrast, the active stylus drives unique modulatedsignals between the tip electrode (antenna) of the stylus and a grid ormatrix of electrodes of the touch-screen sensor (e.g., a digitizersystem) and utilizes sensing methods based on changes in the capacitivecoupling between sensor electrodes. The digitizer system detects atleast one position of the stylus based on the signal emitted and thedetected position provides input to the electronic device (e.g. acomputing device) associated with the digitizer system. The detectedposition may then be interpreted as user commands. Often, the digitizersystem is integrated with a display screen, e.g., to form atouch-sensitive display device.

An active stylus may generate a modulated signal that may be detectableby the digitizer. The signal may be encoded with information such as thedevice identification, operational mode (e.g., writing, erasing),pressure/force information, tilt information, and other information. Theinformation may be allocated to various portions of the signal.

FIG. 1 is a schematic illustration of an example system comprising ahost device (e.g. touch-sensitive display device) 20 and a stylus 10with a driver circuit 120 for applying an output signal to a tipelectrode 100 (output electrode) of the stylus 10 and for receiving aninput signal from the host device via a capacitive coupling between thetip electrode 100 and a touch screen (TS) 200 or other type oftouch-sensitive display of the host device 20. The touch screen 200 iscontrolled by a touch screen controller (TSC) 220 which provides aninterface between a capacitive touch screen sensor array (e.g. a grid ormatrix of electrodes) of the touch screen 200 and the controlled hostdevice 20. The touch screen controller 220 may be connected to the touchscreen sensor array of the touch screen 200 and to the host device 20via a serial connection such as RS232 or universal serial bus (USB). Thecapacitive touch screen sensor array may be placed on a special panelcoated with e.g. indium tin oxide and configured to conduct a continuouselectrical current across the sensor array in order to detect changes incapacitance.

The touch screen controller 220 may be configured to apply a voltageacross a row and column arrangement of the touch sensor array that makesup the touchscreen. The touch sensor array may be scanned by testingrows and columns while they are being stimulated. Testing may involvedetecting the impedance of the circuit by the amplitude of the voltageacross the touch sensor array at each coordinate. This may be achievedby a differential amplifier and an analog-to-digital converter (usuallyof the successive approximation type). This information is passed to thehost device 20 for analysis.

It is noted that, for reasons of brevity, only those system componentsare shown in FIG. 1 and the following FIGS. 2, 4, 6 and 10 , that areuseful for explaining specific example operations of exampleembodiments. Moreover, unless indicated otherwise, components or signalswith identical reference signs have same or similar functionalitiesand/or structures which may be described only once in the presentdisclosure.

FIG. 2 is a schematic circuit diagram of an example driver circuit witha push-pull circuit 122 for applying a dynamically changing voltage Vtip(e.g. a rectangular binary signal) to the tip electrode 100 of thestylus 10 of FIG. 1 .

As an alternative, the driver circuit may as well be provided at one ormore output terminal(s) of the touch screen controller 220 for applyinga voltage to the touch screen sensor array.

The input impedance of the touch screen 200 is represented by acapacitive load C2 and a resistive load R1. The capacitive load C2represents a first capacitance (e.g. about 30 picofarads (pF)) betweenthe tip electrode 100 and ground (reference potential) and a secondcapacitance (e.g. about 30 femtofarads (fF)) between the tip electrodeand the touch sensor array of the touch screen 200. In view of its smallvalue, the second capacitance can be neglected when the capacitive loadC2 is determined. Furthermore, the parallel resistive load R1 is veryhigh and its influence on the dynamic behavior of the impedance in theconsidered frequency range (in the kilohertz (kHz) range up to about 1megahertz (MHz)) of the drive signal (e.g., the tip voltage Vtip)supplied via the tip electrode 100 can therefore be neglected as well.

The push-pull circuit 122 of FIG. 2 consists of two controllableswitching elements SW1, SW2 which are connected between a supplyterminal of a high voltage Vhv and a reference potential (e.g. ground)and which may be implemented as semiconductor switches (e.g.transistors) controlled by a drive control circuit (not shown in FIG. 2).

FIG. 3 is a schematic waveform diagram with example signals of thedriver circuit of FIG. 2 . More specifically, the upper waveform in FIG.3 represents the tip voltage Vtip, the middle waveform represents acontrol signal of the first switching element SW1, and the lowerwaveform represents a control signal of the second switching elementSW2. The control of the first and second switching elements SW1, SW2 isconfigured so that a high control signal closes the correspondingswitching element while a low control signal opens the correspondingswitching element.

As shown in FIG. 3 , the first switching element SW1 is closed and thesecond switching element SW2 is opened during a positive halfwave orpulse of the rectangular tip voltage Vtip, while the first switchingelement SW1 is opened and the second switching element SW2 is closedduring a negative halfwave or pause of the rectangular tip voltage Vtip.Thereby, the capacitive load C2 is charged via the first switchingelement SW1 at the rising edge of the tip voltage Vtip and discharged toground via the second switching element SW2 at the falling edge of thetip voltage Vtip.

Thus, the push-pull circuit 122 is used to generate the tip voltage Vtipwith binary information (e.g. data bits) and a high voltage value (e.g.20 volts (V)) at the tip electrode 100 of the stylus. The tip voltageVtip generates a current through the capacitance between the tipelectrode 100 and the touch screen of the digitizer of the host device.This current can be sensed by the digitizer to determine the location ofthe stylus and/or additional information communicated by the tip voltageVtip.

A high tip voltage Vtip is desirable to increase signal power andefficiency of the driver circuit, because the current through thecapacitance between the tip electrode 100 and the touch screen of thedigitizer of the host device is very low. However, the capacitive loadC2 at the tip electrode 100 causes considerable energy loss through thedischarging of energy stored in the capacitive load C2 to ground atevery falling edge of the tip voltage Vtip, generally at a frequency inthe kHz range. These energy losses depend on the tip voltage Vtip, thevalue of the capacitance and the frequency and can for example amount toabout 30% of total power consumption of the driver circuit.

FIG. 4 is a schematic block diagram of an example driver circuit 120with an energy transfer circuit (PP) 124 for improved power efficiency.The energy transfer circuit 124 is arranged in a path between areference terminal at a reference potential 150 (e.g. ground potential)and an output terminal 140 (e.g. the tip electrode 100 of the stylus 10or an output electrode of the touch screen controller 220 of FIG. 1 ).The driver circuit 120 includes a storage element (SE) 130 (e.g. atleast one inductive element such as an inductor or coil or the like) ora combination of at least one inductive element and at least onecapacitive element (e.g. a capacitor or MOS transistor or varactor orthe like) for temporarily storing charging energy of the capacitive loadC2. The energy transfer circuit (PP) 124 may be composed of anarrangement of at least two switching elements (e.g. semiconductorswitching elements such as transistors, thyristors etc.) and at leasttwo valve elements (e.g. diodes, transistors etc.) and which may beintegrated on a semiconductor chip.

Furthermore, the energy transfer circuit 124 is controlled by controlsignals generated by a driver control circuit (DC) 110 to direct energydischarged from the capacitive load C2 into the storage element 130 andto recharge the capacitive load C2 before the next discharging cycle.The driver control circuit 110 may be implemented by an integrated ordiscrete hardware circuit (e.g. a digital logic circuit, an analogcircuit, an application specific integrated circuit (ASIC) or digitalsignal processor (DSP)) or a software-controlled microcontroller (e.g.central processing unit (CPU)) integrated on the same chip or the samecircuit board as the driver circuit 120 and/or the energy transfercircuit 124.

Thus, the energy transfer circuit 124 is controlled by the drivercontrol circuit 110 to discharge and recharge the capacitive load C2(e.g., an output capacitor) and facilitate the transition from low tohigh value of the voltage at the output terminal 100 to thereby reduceenergy loss and increase efficiency of the driver circuit 120.

FIG. 5 is a schematic flow diagram of an example driver controlprocedure for controlling an energy transfer circuit with a switchedenergy storage element (e.g. the energy storage element 130 of FIG. 4 ).

The driver control procedure may be implemented by a software routinestored in a memory and used to control a processor or controller in adriver control circuit (e.g. the driver control circuit 110 of FIG. 4 )to apply control signals to control the energy transfer circuit (e.g.the energy transfer circuit 124 of FIG. 4 ).

At S310 the procedure controls the energy transfer circuit to charge thecapacitive load C2 at a rising edge of the output voltage at an outputterminal of the driver circuit, e.g., the output terminal (tipelectrode) 100 of the driver circuit 120 of the stylus 10 and/or anoutput terminal of a driver circuit of the touch screen controller 220of FIG. 1 . Then, at S320, the energy transfer circuit is controlled todischarge the capacitive load C2 into the storage element of the drivercircuit at a falling edge of the output voltage to temporarily storeenergy in the storage element.

At S330, the energy transfer circuit may optionally be controlled tointermittently connect (pull down) the output terminal to a referencepotential (e.g. ground potential) of the output voltage for apredetermined time period (e.g. between the falling edge and the nextrising edge of the output voltage) in order to reduce crosstalk and/orother noise.

At S340, the energy transfer circuit is controlled to recharge thecapacitive load C2 with the energy stored in the storage element priorto the next rising edge of the output voltage used for further chargingof the capacitive load to a maximum value of the output voltage.

With the above driver control procedure, the charge stored in thecapacitive load is temporarily stored in the storage element andretransferred to the capacitive load C2 to thereby enhance efficiencyand reduce power loss of the driver circuit.

Non-limiting examples of possible arrangements of switching elements(e.g., SW1 to SW4) and valve elements (e.g. diodes D1 and D2) in anenergy transfer circuit of a driver circuit of a stylus (e.g. the drivercircuit 120 of the stylus 10 of FIG. 1 ) are described in more detailwith reference to FIGS. 6 to 14 . The switching elements shown in FIGS.6 and 10 are optionally implemented as controllable semiconductorswitches (e.g. PMOS, NMOS, or CMOS transistors).

FIG. 6 is a schematic circuit diagram of a driver circuit with a firstmore detailed example of an energy transfer circuit configured as anenhanced push-pull circuit 126 for improved efficiency in case of anoutput voltage with a voltage swing around zero (e.g. −9V to +11V or−18V to +20V), e.g., positive and negative halfwaves of the rectangularoutput voltage of the driver circuit.

In the example of FIG. 6 , a parallel connection of second and thirdswitching elements SW2, SW3 with respective serially connectedantiparallel diodes D1, D2 is serially connected between a firstswitching element SW1 and an inductor L1 in a circuit path between aterminal of a supply voltage Vhv and a reference potential 150 (e.g.ground potential) of the tip voltage Vtip at the tip electrode 100 ofthe stylus. The tip electrode 100 is connected between the firstswitching element SW1 and the parallel connection with the second andthird switching elements SW2, SW3 and the first and second diodes D1,D2.

It is noted that the switching state of the first to third switchingelements SW1 to SW3 is controllable via respective control signalsapplied by a driver control circuit (e.g. DC 110, FIG. 4 ) not shown inFIG. 6 .

Furthermore, the inductor L1 may be provided as an external circuitelement while the first to third switching elements SW1 to SW3 and thefirst and second diodes D1, D2 may be integrated on a chip, or on one ormore chip modules, of the enhanced push-pull circuit 126 or the wholedriver circuit.

Moreover, it is noted that with the driver circuit of FIG. 6 a voltagearound e.g. 10V could for example be generated if the inductor L1 wasconnected to a reference potential 150 of e.g. 10V instead of groundpotential. Of course, other values of the reference potential 150 couldbe applied as well.

FIG. 7 is a schematic waveform diagram which shows from top to bottomthe waveforms of the tip voltage Vtip, a control signal of the firstswitching element SW1, a control signal of the second switching elementSW2, a control signal of the third switching element SW3 and theinductor current IL1 of the first example of the enhanced push-pullcircuit of FIG. 6 . The value and sign of the control signals isselected based on the respective types of the first to third switchingelements SW1 to SW3.

At a falling edge, e.g., a transition of the rectangular tip voltageVtip from high to low voltage, the second switching element SW2 iscontrolled to be closed (and the current path is opened) for apredetermined time period (positive pulse of the respective controlsignal) until the tip voltage Vtip changes to the low voltage todischarge the capacitive load C2 via the first diode D1 (valve element)into the inductor L1. This is indicated by a positive pulse in thewaveform of the inductor current IL1. Due to the generated magneticfield (stored magnetic energy) of the inductor L1, the current IL1through the inductor L1 continues to flow after complete discharge ofthe capacitive load so as to pull the potential at the tip electrode 100to a negative value. The valve effect of the first diode D1 in thecurrent path prevents a change of the flow direction of the current IL1to thereby suppress an oscillation between the inductor L1 and thecapacitive load C2.

Shortly before the subsequent rising edge, e.g., a transition of the tipvoltage Vtip from low to high voltage, the capacitive load C2 is chargedwith a negative voltage during the discharging phase via the secondswitching element SW2 and a closing (e.g. positive pulse of therespective control signal) of the third switching element SW3 causes thecapacitive load C2 to discharge into the inductor L1 which pulls the tipvoltage Vtip to a positive value. This is indicated by a negative pulsein the waveform of the inductor current IL1. The timing of the switchingof the third switching element SW3 is selected according to the desiredwaveform of the tip voltage. After that, the first switching element SW1is closed at the rising edge of the tip voltage Vtip so that thecapacitive load C2 is charged via the first switching element SW1 to thevalue of the supply voltage Vhv, while the second and third switchingelements SW2 and SW3 are open. As an example, the second switchingelement SW2 can be open (non-conductive) for the whole rising edge ofthe tip voltage Vtip. During the rising edge, the third switchingelement SW3 can be closed (conductive). The first switching element SW1can be closed (conductive) after the inductor L1 has charged thecapacitive load C2. The second diode D2 allows that the first switchingelement SW1 can be closed while the third switching element SW3 isclosed, but this is an optional feature.

FIG. 8 is a waveform diagram with different measured waveforms of thetip voltage Vtip, control signals of the switching elements SW1 and SW2and the inductor current IL1 through the inductor L1 obtained as aresult of a simulation of the first example of the enhanced push-pullcircuit (FIG. 6 ) at a falling edge of the tip voltage Vtip of thestylus. In this example, the tip voltage Vtip changes in a substantiallyrectangular manner between about +20V and about −17V.

As shown in FIG. 8 , the inductor current IL1 is generated as a positivepulse at the falling edge of the tip voltage Vtip and includes somesmall parasitic oscillations until the second switching element SW2 issubsequently opened. These small parasitic oscillations are alsoreflected at the beginning of the negative halfwave of the tip voltageVtip while the second switching element SW2 is closed and coupling theinductor L1 with the tip electrode 100. The small parasitic oscillationsare caused by a non-ideal behaviour of the diode D1 and therefore dependon the circuit design. The first and second diodes D1, D2 may bereplaced by respective control loops that close the correspondingswitching element when the current has reached zero.

FIG. 9 is a waveform diagram with the different measured waveforms ofthe tip voltage Vtip, control signals of the switching elements SW1 andSW3 and the inductor current IL1 through the inductor L1 of thesimulation of the first example of the enhanced push-pull circuit at arising edge of the tip voltage Vtip of the stylus.

As shown in FIG. 9 , the inductor current IL1 is generated as a negativepulse at the rising edge of the tip voltage Vtip and includes some smallparasitic oscillations (due to non-perfect diode behavior) until thethird switching element SW3 is subsequently opened. These smallparasitic oscillations are also reflected at the beginning of thepositive halfwave of the tip voltage Vtip while the third switchingelement SW3 is closed and coupling the inductor L1 with the tipelectrode 100. However, in this example, the re-charging of thecapacitive load C2 with the stored charge of the inductor L1 by theinductor current IL1 during the closed state of the third switchingelement SW3 is not sufficient to reach the maximum value (+20V) of thetip voltage Vtip again, due to remaining energy losses during thecharging and re-charging process via the inductor L1. As can be gatheredfrom the waveform diagram, the capacitive load C2 is first re-charged toan intermediate maximum value of about +15V. Thereafter, the originalmaximum value (+20V) of the tip voltage Vtip is reached when the firstswitching element SW1 is subsequently closed and the capacitive load C2is charged to the full value of the supply voltage Vhv. In this way, thesupply voltage Vhv is used to charge the capacitive load C2 from +15V to+20V, rather than from 0V to +20V as may be the case with a conventionalpush-pull circuit. Thus, for the same peak-to-peak value of the tipvoltage Vtip, the conventional push-pull circuit of FIG. 2 would requireabout four times more power than the exemplary driver circuit of FIG. 10. From a design perspective, the driver circuit and its supply voltageVhv can thus be designed to meet the requirements for the peak-to-peakvalue of the tip voltage Vtip at muss less power loss.

FIG. 10 is a schematic example circuit diagram of a driver circuit witha more detailed second example of an energy transfer circuit implementedas an enhanced push-pull circuit 128 for improved efficiency in case ofan output voltage that stays in a positive voltage range (e.g. 0V to+20V) above ground potential with a resulting steady state. Furthermore,the fact that the tip voltage Vtip and other circuit voltages arepositive voltages above ground potential is advantageous for the chipdesign. Such a positive voltage range reduces complexity of input/output(I/O) circuit measures required for protection against electrostaticdischarge (ESD).

Moreover, a reduced crosstalk can be achieved by providing an additionalpath to the reference potential 150 (e.g. ground potential) via anadditional fourth switching element SW4 which may also be controlled bya driver control circuit (not shown in FIG. 10 ) to couple the tipelectrode 100 for a temporary period to the reference potential 150.

In this example, similar to FIG. 6 , the parallel connection of thesecond and third switching elements SW2, SW3 with respective seriallyconnected diodes D1, D2 is serially connected between the firstswitching element SW1 and the inductor L1 in the circuit path betweenthe terminal of a supply voltage Vhv and the reference potential (e.g.ground potential) of the tip voltage Vtip at the tip electrode 100 ofthe stylus. Furthermore, the tip electrode 100 is again connectedbetween the first switching element SW1 and the parallel connection withthe second and third switching elements SW2, SW3 and the first andsecond diodes D1, D2.

However, unlike in the first example of FIG. 6 , the fourth switchingelement SW4 is connected between the tip electrode 100 and the referencepotential 150. Furthermore, an additional storage capacitor C3 isconnected in series between the inductor L1 and the reference potential150 to store a steady-state DC component of the rectangular tip voltageVtip. The additional storage capacitor C3 provides an advantage in thatan additional voltage supply structure (e.g. rail) and related costs canbe prevented. In the example of FIG. 10 , the additional storagecapacitor C3 is charged to one half of the supply voltage Vhv in thesteady state.

It is noted that the switching state of the first to fourth switchingelements SW1 to SW4 is again controllable via respective control signalsapplied by a driver control circuit (e.g. DC 110 of FIG. 4 ) not shownin FIG. 10 . These control signals are again adapted to the respectivetypes of the switching elements SW1 to SW4.

Furthermore, the inductor L1 and/or the storage capacitor C3 may beprovided as an external circuit element while the first to thirdswitching elements SW1 to SW4 and the first and second diodes D1, D2 maybe integrated on a chip, or on one or more chip modules, of the enhancedpush-pull circuit 128 or the whole driver circuit.

FIG. 11 is a schematic waveform diagram which shows from top to bottomthe waveforms of the tip voltage Vtip, a control signal of the firstswitching element SW1, a control signal of the second switching elementSW2, a control signal of the fourth switching element SW4, a controlsignal of the third switching element SW3 and the inductor current IL1of the second example of the enhanced push-pull circuit of FIG. 10 .

At a falling edge, e.g., the transition of the rectangular tip voltageVtip from high to low voltage, the second switching element SW2 iscontrolled to be closed (and the current path is opened) for apredetermined time period (positive pulse of the respective controlsignal) less than half of the period of the tip voltage Vtip todischarge the capacitive load C2 via the first diode D1 (an examplevalve element) through the inductor L1 into the storage capacitor C3.This is indicated by a positive pulse in the waveform of the inductorcurrent IL′. Due to the use of the inductor L1, there is only a littleremaining energy loss caused by parasitic resistances of the switchingelements SW1 to SW4, diodes D1 and D2, the inductor L1 and the storagecapacitor C3. Without the inductor L1, not all of the energy stored inthe capacitive load C2 will be transferred to the storage capacitor C3so that power loss will increase.

When the fourth switching element SW4 is then closed for a predeterminedtime period before the subsequent rising edge of the tip voltage Vtip,the tip electrode 100 is pulled down and held at the reference potential150 (e.g. ground potential) to thereby prevent crosstalk and/or othernoise or interference.

The valve effect of the first diode D1 in the current path prevents achange of the flow direction of the current IL1 to thereby suppress anoscillation between the inductor L1, the storage capacitor C3 and thecapacitive load C2.

Shortly before the subsequent rising edge, e.g., the transition of thetip voltage Vtip from low to high voltage, a closing (positive pulse ofthe respective control signal) of the third switching element SW3 undercontrol of the driver control circuit causes a transition of the chargeof the storage capacitor C3 via the second diode D2 and the thirdswitching element SW3 into the capacitive load C2. This is indicated bya negative pulse in the waveform of the inductor current ILL After that,the first switching element SW1 is closed at the rising edge of the tipvoltage Vtip so that the capacitive load C2 is charged via the firstswitching element SW1 to the value of the supply voltage Vhv, while thesecond, third and fourth switching elements SW2 to SW4 are open.

FIG. 12 is a waveform diagram with different measured waveforms of thetip voltage Vtip, a voltage VC3 across the storage capacitor C3, controlsignals of the switching elements SW1, SW2 and SW4, and the inductorcurrent IL1 through the inductor L1 and the storage capacitor C3obtained as a result of a simulation of the second example of theenhanced push-pull circuit at a falling edge of the tip voltage Vtip ofthe stylus. In this example, the tip voltage Vtip changes in asubstantially rectangular manner between about +20V and 0V.

As shown in FIG. 12 , the inductor current IL1 is generated as apositive pulse at the falling edge of the tip voltage Vtip and includessome small parasitic oscillations until the second switching element SW2is opened again. These small parasitic oscillations are also reflectedat the tip voltage Vtip and the voltage VC3 across the storage capacitorC3 which is stored to a value of about +13V during its fully chargedstate.

FIG. 13 is a waveform diagram with the different measured waveforms ofthe tip voltage Vtip, the voltage VC3 across the storage capacitor C3,the control signals of the switching elements SW1 and SW3, and theinductor current IL1 through the inductor L1 and the storage capacitorC3 of the simulation of the second example of the enhanced push-pullcircuit at a rising edge of the tip voltage Vtip of the stylus.

As shown in FIG. 13 , the inductor current IL1 is generated as anegative pulse at the rising edge of the tip voltage Vtip and includessome small oscillations until the third switching element SW3 is openedagain. These small oscillations are also reflected at the tip voltageVtip and the voltage VC3 across the storage capacitor C3 which isde-charged to a value of about +7V during its de-charged state. However,the re-charging of the capacitive load C2 with the stored charge of theinductor L1 by the inductor current IL1 during the closed state of thethird switching element SW3 is not sufficient to reach the maximum value(+20V) of the tip voltage Vtip again due to remaining energy lossesduring the charging and re-charging process via the inductor L1. As canbe gathered from the waveform diagram, the capacitive load C2 is nearlycompletely re-charged to the maximum supply voltage value of about +20Vbased on the charge of the storage capacitor C3. This illustrates therelatively little amount of energy loss in this example embodiment thatincludes the storage capacitance C3.

FIG. 14 is a waveform diagram with the different measured waveforms ofthe tip voltage Vtip, the voltage VC3 across the storage capacitor C3,the control signals of the switching elements SW1 to SW4, and theinductor current IL1 through the inductor L1 and the storage capacitorC3 of the second example of the enhanced push-pull circuit over a longertime period until a steady state is reached.

As illustrated in FIG. 14 , in some implementations it takes multiple(e.g., three) charging/de-charging cycles of the storage capacitor C3until a steady state with a constant maximum value of the voltage VC3across the storage capacitor C3 has been reached. In the example of FIG.10 , the storage capacitance C3 is assumed to be bigger than thecapacitive load C2. Thus, during a small initial time period until thesteady state has been reached, it will take some time until sufficientenergy has been transferred from the smaller capacitive load C2 tocharge the larger storage capacitor C3 to its steady-state voltage (e.g.10 y). Thereafter, during the steady state, the voltage at the storagecapacitor C3 stays around its steady-state value (under the assumptionthat in average the high time and the low time of the binary waveform ofthe tip voltage Vtip is the same, which is true for any digitalcommunication).

An apparatus and method for a voltage driver circuit have beendescribed, where a path between a ground node and an output nodeincludes an inductor and/or another storage element and an energytransfer circuit composed of at least two switching elements (e.g.,transistors) and at least two valve elements (e.g. diodes). The energytransfer circuit operates to discharge a capacitive load (e.g. outputcapacitor) into the storage element and facilitate transition from lowto high voltage while increasing efficiency. An additional path toground may be included via another switching element to preventcrosstalk while holding the output at a ground potential.

With the proposed new designs of the driver circuit with the energytransfer circuit according to the above sample embodiments, the totalpower consumption of the driver circuit can be reduced substantially(e.g. from about 2.5 mW to about 0.2 mW) to thereby increase batterylifetime and efficiency. Furthermore, compared to alternative solutions,such as Collpits oscillators or adiabatic circuits, the proposed drivercircuits can be used in connection with a wider range of frequencies andhigher voltages, allow full control of high and low time of the driveroutput voltage and therefore enable digital data at base band level andadvanced modulation in frequency and phase. Higher voltages allowsmaller electrodes (e.g. at the tip of the stylus or the touch sensor ofthe digitizer) with less capacitive load and thus more precisedetermination of the stylus location.

It will be appreciated that the above embodiments have been described byway of example only.

More generally, according to one aspect (A1) disclosed herein, there isprovided a driver circuit (e.g., driver circuit 120, FIG. 4 ) forsupplying a drive signal via an output terminal (e.g., output terminal100, FIG. 4 ) to a capacitive load (e.g., C2, FIG. 4 ), wherein thedriver circuit comprises:

an energy storage element (e.g., SE 130, FIG. 4 ); and

an energy transfer circuit (e.g., PP 124, FIG. 4 ) configured totransfer energy from the capacitive load to the energy storage elementat a first edge of the drive signal and to transfer energy from theenergy storage element to the capacitive load at a second edge of thedrive signal.

(A2) In embodiments of A1, the energy transfer circuit is controlled tointermittently connect the output terminal to a reference potential(e.g., ground potential 150 in FIGS. 4, 6 and 10 ) of the output voltagefor a predetermined time period.

(A3) In embodiments of A1 or A2, the storage element comprises at leastone inductive element (e.g., inductor L1, FIG. 6 ), at least onecapacitive element (e.g., capacitor C3, FIG. 10 ), or a combination ofat least one inductive element and at least one capacitive element.

(A4) In embodiments of A1-A3, the energy transfer circuit comprises anarrangement of at least two switching elements (e.g., SW2 and SW3, FIG.6 ) and at least two valve elements (e.g., D1 and D2, FIG. 6 ).

(A5) In embodiments of A1-A4, the energy transfer circuit comprises aparallel connection of second and third switching elements withrespective serially connected antiparallel valve elements (e.g., D1 andD2, FIG. 6 ), the parallel connection being serially connected between afirst switching element and the energy storage element in a circuit pathbetween a terminal of a supply voltage and a reference potential of thedrive signal at the output terminal, wherein the output terminal isconnected between the first switching element and the parallelconnection.

(A6) In embodiments of A5, the second switching element is controlled tobe closed for a predetermined time period in conjunction with the firstedge (e.g., a falling edge) of the drive signal to discharge thecapacitive load via a first valve element (e.g., D1 FIG. 6 ) into thestorage element.

(A7) In embodiments of A5 or A6, the third switching element iscontrolled to be closed in conjunction with the second edge (e.g., arising edge) of the drive signal to cause the capacitive load todischarge into the storage element, and wherein thereafter the firstswitching element is closed and the third switching element is opened inconjunction with the second edge of the drive signal so that thecapacitive load is charged via the first switching element, while thesecond and third switching elements are open.

(A8) In embodiments of A4-A7, a fourth switching element (e.g., SW4,FIG. 10 ) is connected between the output terminal and the referencepotential.

(A9) In embodiments of A1-A8, the storage element comprises a seriesconnection of an inductor and a capacitor (e.g., capacitor C3, FIG. 10), and wherein the capacitor is used as energy storage.

(A10) In embodiments of A1-A9, the energy transfer circuit is integratedon a chip or one or more chip modules, and wherein the storage elementis arranged as a non-integrated external circuit element.

(A11) In embodiments of A4-A10, the third switching element iscontrolled to be closed in conjunction with the second edge (e.g., arising edge) of the drive signal to cause a charge of the capacitiveload to a first level via a transition of energy from the energy storageelement (e.g., C3 in FIG. 10 ) via a second valve element (e.g., D2 inFIG. 10 ) and the third switching element into the capacitive load, andwherein thereafter the first switching element is controlled to beclosed in conjunction with the second edge (e.g., a rising edge) of thedrive signal so that the capacitive load is charged from the first levelto a second level via the first switching element.

According to another aspect disclosed herein, there is provided a stylus(e.g., stylus 10, FIG. 1 ) comprising a driver circuit according to anyembodiment disclosed herein (e.g., A1-A11), wherein the driver circuitis configured to supply the drive signal to a tip electrode of thestylus.

According to another aspect disclosed herein, there is provided a hostdevice (e.g., host device 20, FIG. 1 ) comprising a driver circuitaccording to any embodiment disclosed herein (e.g., A1-A11), wherein thedriver circuit is configured to supply the drive signal to a sensorarray of a touch-sensitive display.

According to another aspect disclosed herein, there is provided a methodof supplying a drive signal via an output terminal to a capacitive load,wherein the method comprises controlling an energy transfer circuit totransfer energy from the capacitive load to an energy storage element ata first edge of the drive signal and to transfer energy from the energystorage element to the capacitive load at a second edge of the drivesignal. In embodiments, the method comprises controlling the push-pullcircuit according to any embodiments disclosed here (e.g., A1-A11).

According to another aspect disclosed herein, there is provided acomputer program embodied on computer-readable storage (e.g., withinmemory such as random access memory (RAM) or read only memory (ROM)) andcomprising code configured so as when run on one or more processors toperform the method of any embodiment disclosed herein.

Examples and embodiments described herein may be implemented as logicalsteps in one or more computer systems. The logical operations may beimplemented (1) as a sequence of processor-implemented steps executingin one or more computer systems and (2) as interconnected machine orcircuit modules within one or more computer systems. The implementationis a matter of choice, dependent on the performance requirements of thecomputer system used for implementation. Accordingly, logical operationsmaking up examples or embodiments described herein may be referred tovariously as operations, steps, objects, or modules. Furthermore, itshould be understood that logical operations may be performed in anyorder, adding and omitting as desired, unless explicitly claimedotherwise or a specific order is inherently necessitated by the claimlanguage.

Other variants and applications of the disclosed techniques may becomeapparent to a person skilled in the art once given the presentdisclosure. The scope of the present disclosure is not limited by theabove-described embodiments but only by the accompanying claims.

1. A driver circuit for supplying a drive signal via an output terminalto a capacitive load, wherein the driver circuit comprises: an energystorage element; and an energy transfer circuit configured to transferenergy from the capacitive load to the energy storage element at a firstedge of the drive signal and to transfer energy from the energy storageelement to the capacitive load at a second edge of the drive signal. 2.The driver circuit of claim 1, wherein the energy transfer circuit iscontrolled to intermittently connect the output terminal to a referencepotential of the output voltage for a predetermined time period.
 3. Thedriver circuit of claim 1, wherein the storage element comprises atleast one inductive element or a combination of at least one inductiveelement and at least one capacitive element.
 4. The driver circuit ofclaim 1, wherein the energy transfer circuit comprises an arrangement ofat least two switching elements and at least two valve elements.
 5. Thedriver circuit of claim 4, wherein the energy transfer circuit comprisesa parallel connection of second and third switching elements withrespective serially connected antiparallel valve elements, the parallelconnection being serially connected between a first switching elementand the energy storage element in a circuit path between a terminal of asupply voltage and a reference potential of the drive signal at theoutput terminal, wherein the output terminal is connected between thefirst switching element and the parallel connection.
 6. The drivercircuit of claim 5, wherein the second switching element is controlledto be closed for a predetermined time period in conjunction with thefirst edge of the drive signal to discharge the capacitive load via afirst valve element into the storage element.
 7. The driver circuit ofclaim 5, wherein the third switching element is controlled to be closedin conjunction with the second edge of the drive signal to cause thecapacitive load to discharge into the storage element, and whereinthereafter the first switching element is closed and the third switchingelement is opened in conjunction with the second edge of the drivesignal so that the capacitive load is charged via the first switchingelement, while the second and third switching elements are open.
 8. Thedriver circuit of claim 5, wherein a fourth switching element isconnected between the output terminal and the reference potential. 9.The driver circuit of claim 5, wherein the storage element comprises aseries connection of an inductor and a capacitor, and wherein thecapacitor is used as energy storage.
 10. The driver circuit of claim 1,wherein the energy transfer circuit is integrated on a chip or one ormore chip modules, and wherein the storage element is arranged as anon-integrated external circuit element.
 11. The driver circuit of claim10, wherein the third switching element is controlled to be closed inconjunction with the second edge of the drive signal to cause a chargeof the capacitive load to a first level via a transition of energy fromthe energy storage element via a second valve element and the thirdswitching element into the capacitive load, and wherein thereafter thefirst switching element is controlled to be closed in conjunction withthe second edge of the drive signal so that the capacitive load ischarged from the first level to a second level via the first switchingelement.
 12. A stylus comprising a driver circuit according to claim 1,wherein the driver circuit is configured to supply the drive signal to atip electrode of the stylus.
 13. A host device comprising a drivercircuit according to claim 1, wherein the driver circuit is configuredto supply the drive signal to a sensor array of a touch-sensitivedisplay.
 14. A method of supplying a drive signal via an output terminalto a capacitive load, wherein the method comprises controlling an energytransfer circuit to transfer energy from the capacitive load to anenergy storage element at a first edge of the drive signal and totransfer energy from the energy storage element to the capacitive loadat a second edge of the drive signal.
 15. A computer program embodied oncomputer-readable storage and comprising code configured so as when runon one or more processors to perform the method of claim 14.